The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential, and more particularly to a control system in which output torque of a transmission is unequally distributed to front wheels and rear wheels in accordance with driving conditions.
In a full-time four-wheel drive motor vehicle with the central differential, a fluid operated multiple-disk friction clutch is provided for restricting differential operations. The torque distributed to the front wheels and the rear wheels is controlled by controlling clutch torque of the friction clutch, thereby improving straight-ahead stability, accelerability and driveability. In order to widely control the distribution of the torque, it is necessary that the central differential is arranged to unequally distribute the torque to the front wheels and the rear wheels. There are two methods in the unequal distributing system. One of the methods is a front-weighted distribution which is weighted to the front wheels, and the other is a rear-weighted distribution which is weighted to the rear wheels. In the front-wheel weighted system, larger torque is transmitted to the front wheels. Accordingly, in the front-weighted system, although the vehicle has good straight-ahead stability, it is inferior in steerability because of drifting out of a course. In the rear-wheel weighted system, larger torque is transmitted to the rear wheels. The vehicle has good steerability and driveability on a dry road. However, the vehicle is liable to spin on a slippery road.
On the other hand, a disadvantage of the four-wheel drive vehicle is that all the four wheels may slip (slipping spin) at the same time, causing difficulty in driving.
In order to ensure driving stability of the vehicle, the torque to the rear wheels is set to a larger value than that to the front wheels, so that the rear wheels may slip first. Thus, the vehicle can be safely driven without a sudden change of driving stability (i.e. there is no slipping of the four wheels.).
Since the driving force (acceleration force) is distributed to the four wheels, each wheel can sustain a larger side force. Thus, in order to ensure driving stability in the rear-wheel weighted system, it is necessary to keep the sustainable side force of the rear wheels large.
The driving force and the side force change with the slip ratio of the wheel to the ground (slip ratio is described hereinafter in detail). The slip ratio changes in accordance with the condition of the road surface and the friction coefficient of the tire. In particular, if the slip ratio is larger than a predetermined value (10 to 20%), the driving force and the side force remarkably decrease, which results in deterioration of the driving stability. Accordingly, the slip ratio of the rear-wheels is controlled so as not to exceed the predetermined value. If the system operates to increase the torque distributed to the front wheels in accordance with the reduction of the sustainable side force, the slipping of the rear wheels is prevented to ensure the driving force. Therefore, it is desirable to control the torque distribution based on the rear-wheel slip ratio.
Japanese Patent Applications Laid-Open 62-55228, 62-261539 and 63-8026 disclose four-wheel drive control systems in which a central differential comprising a simple planetary gear device is provided. A standard torque to the front and rear wheels is unequally distributed at a ratio determined by a difference between the pitch circles of gears of the planetary gear device. Torque capacity of the clutch is controlled in accordance with the difference between the speeds of the front wheels and the rear wheels, or with a predetermined value corrected by vehicle speed and steering angle.
It is not advantageous to control the torque distribution under all driving conditions in the same manner. It is desirable to set the torque distribution ratio at a predetermined fixed value under a predetermined driving condition.
Japanese Patent Application Laid-Open 62-283021 discloses a control system in which a torque for restricting the differential operation of the central differential is provided for improving the response of the control operation. Japanese Patent Application Laid-Open 63-57334 discloses a control system in which the torque for operating a differential operation restricting clutch is kept to a predetermined value at deceleration and a predetermined low value when an antilock brake system (ABS) is operated.
However, in the conventional systems, the standard torque distribution ratio determined by the ratio of the pitch circles can not be changed, unless the diameters of the gears change. However, it is difficult to change the diameters because of a limitation of space. Since the distribution of torque to the rear wheels can not be set to a large value, the control range of the torque distribution is small.
Further, the torque distribution is controlled by restricting the differential operation of the central differential. Therefore, slip conditions of the wheels on the road surface can not be detected so that it is impossible to actively control the torque distribution in accordance with the slip conditions. In addition, if the vehicle is driven on a road surface having a low coefficient of friction under slipping conditions and the speed difference is small, the torque distribution can not be controlled.
Further, in the conventional systems for the fixed torque distribution control, the control range of the driving conditions is limited to a particular condition such as deceleration. Therefore, it is desirable to control the distribution at a fixed ratio in a wide range of driving conditions including acceleration and steering operation.